1. Field of the Invention
This invention relates to the preparation of cyclic urea prepolymer-modified melamine-containing resins, to their use in binders and to cellulosic-based composite products prepared using the binders. More particularly, the invention relates to a melamine-containing resin modified a with cyclic urea prepolymer comprising urea, formaldehyde, and ammonia or a primary amine which provides useful binders alone or with other resins such as urea-formaldehyde and phenol-formaldehyde based resins. Products prepared with the binders include wood composites such as particleboard and medium density fiberboard.
2. Description of Related Art
Urea-formaldehyde resins are used in the preparation of wood composites such as particleboard and medium density fiberboard because of their processing advantages and low cost relative to other typical wood adhesives. Urea-formaldehyde resins are typically prepared by reacting urea and formaldehyde to form various methylolated ureas and their condensation products in ratios that depend on the temperature, pH and time for the reaction.
The use of urea-formaldehyde resins is limited to applications where exterior durability is not required. Melamine has been included in urea-formaldehyde binders to improve the moisture resistance and thickness swelling performance of composites made with these binders. The melamine can be incorporated into wood composites in the form of a melamine-urea-formaldehyde resin, a melamine-formaldehyde resin, or as a blend of a urea-formaldehyde resin and melamine-formaldehyde resin. (U.S. Pat. Nos. 3,827,995 and 3,979,492) However, the addition of melamine adds significant cost to the composite.
Further, when urea-formaldehyde resins are used as a binder component they release formaldehyde into the environment during cure. Formaldehyde also can be released subsequently from the cured resin, particularly when the cured resin is exposed to acidic environments. Such formaldehyde release is undesirable, particularly in enclosed environments. In such environments, formaldehyde is inhaled by workers and comes into contact with the eyes, the mouth, and other parts of the body. Formaldehyde is malodorous and is thought to contribute to human and animal illness.
Various techniques have been used to reduce formaldehyde emission from urea-formaldehyde resins. Use of formaldehyde scavengers and various methods for resin formulation, including addition of urea as a reactant late in the resin formation reaction, are techniques often used to reduce formaldehyde emission. However, use of formaldehyde scavengers often is undesirable, not only because of the additional cost, but also because it affects the characteristics, or properties, of the resin. For example, using ammonia as a formaldehyde scavenger often reduces the resistance of the cured resin to hydrolysis (degradation). Later addition of urea to reduce free formaldehyde concentration in the resin generally yields a resin that must be cured at a relatively low rate to avoid smoking. Resin stability also can be adversely affected by such treatments. Melamine has also been used to prepare resins which have low formaldehyde emissions (U.S. Pat. No. 5,681,917)
Therefore, a binder which provides the advantages of melamine, which include reduced thickness swell, increased moisture resistance, and minimum formaldehyde emissions, at a reduced cost is desired.
The present invention is directed to melamine-containing wood binders that have been modified by replacing part of the melamine with a cyclic urea-formaldehyde prepolymer.
In accordance with the present invention, the cyclic urea prepolymer modifies melamine-containing resins and may be a cold blended additive to other binders such as urea-formaldehyde resins, phenol-formaldehyde resins or melamine-containing resins, or may be a coreactant added during the preparation of the binder.
The present invention is directed to a binder comprising a cyclic urea prepolymer; melamine or a melamine-containing resin; and at least one resin selected from the group consisting of urea-formaldehyde (UF), phenol-formaldehyde (PF), phenol-urea-formaldehyde (PUF), melamine-formaldehyde (MF), phenol-melamine-formaldehyde (PUF), melamine-urea-formaldehyde (MUF), and phenol-melamine-urea-formaldehyde (PMUF); wherein the cyclic urea prepolymer has mole ratios of urea:formaldehyde:ammonia or a primary amine between about 0.1 to 1.0:0.1 to 3.0:0.1 to 1.0, and wherein the cyclic urea prepolymer, the melamine or the melamine-containing resin, and the at least one resin are blended, coreacted, or both.
The present invention is further directed to consolidated wood products prepared with a substrate and a binder as defined above. In particular, the present invention is directed to particle board and medium density fiberboard.
The present invention is also directed to a consolidated wood product comprising a substrate and a wood binder comprising a cyclic urea prepolymer and melamine or a melamine-containing resin; wherein the cyclic urea prepolymer has mole ratios of urea:formaldehyde:ammonia or a primary amine between about 0.1 to 1.0:0.1 to 3.0:0.1 to 1.0, and wherein the cyclic urea prepolymer and melamine or the melamine-containing resin are blended or coreacted to form a cyclic urea prepolymer modified melamine-containing resin